In inks usable for inkjet printers, a pigment ink has been recently used in order to improve water resistance and light fastness.
The pigment ink has been generally prepared by dispersing a pigment in an organic solvent or a liquid mixture of water and an organic solvent in a low solid matter concentration of 10 to 40% by weight or so. Japanese Patent Publication Nos. Hei 8-183920A and Hei 8-218013A disclose that a sand-mill, a beads-mill, a high-pressure homogenizer or the like is used as a dispersing device in the dispersion.
It has been desired to develop a pigment ink having a dispersion particle diameter as small as possible since the pigment ink is inferior in gloss to a dye ink, and being more excellent in filterability with a filter since the pigment ink is inferior in filterability to the dye ink.
However, in the method of dispersing a pigment in a low solid matter concentration as described in the above literatures, the pigment cannot be sufficiently atomized, and filterability is worsened when a pigment having a strong aggregating force of primary particles is used or when the dispersion on the level of the primary particles is necessitated.
On the other hand, as a process of using a solid color compound, Japanese Patent Publication Nos. Hei 10-88042A and 2001-271008A disclose a process comprising mixing or dispersing a solid color compound obtained by kneading a mixture of a molten resin or an organic solvent solution, having an acid value with a pigment, water, an organic solvent and a base. However, in this process, the dispersion into water and the organic solvent may become insufficient in some cases since the solid matter concentration of the solid color compound is very high and the resin is not neutralized.